1. Field of the Invention
The present invention relates to an analog signal synthesizing system in PCM and more particularly to such a system capable of reading PCM data corresponding to plural channels from a waveform memory and synthesizing an analog signal from such PCM data.
2. Description of the Prior Art
PCM has various superior characteristics such as very high resistance to noise, very high resistance to interference with the adjacent channel and others. Therefore, PCM has been currently utilized in various broadly widened applications such as synthesizers, musical compact disc devices, PCM communication systems and others.
FIG. 8 shows the principle of PCM wherein it is supposed that an analog signal such as voice is converted into PCM signal. As shown in FIG. 8A, an analog signal is sampled with a predetermined sampling frequency to form a PAM wave as shown in FIG. 8B. Such a PAM wave is quantized and encoded to provide PCM data.
If the PCM data so obtained are desired to re-convert into an analog signal, they are first decoded to form the PAM wave as shown in FIG. 8B. The PAM wave is then passed through a low-pass filter to reproduce a signal wave having the original signal waveform.
When the analog signal is sampled with the frequency f.sub.s, there is obtained such a waveform spectra as shown in FIG. 9. In FIG. 9, the hatched part is a spectra in the original analog signal. If such a sampling is carried out, there are created a plurality of aliasing noises at locations integer times the sampling frequency, that is, f.sub.s, 2f.sub.s, 3f.sub.s and so on. These aliasing noises will be superposed over the waveform spectra in the present signal if the sampling frequency f.sub.s is too low. Thus, it becomes impossible to faithfully reproduce the original analog signal.
If the sampling frequency f.sub.s is too high, however, the amount of data to be handled increases to make the data processing very cumbersome.
In order to faithfully reproduce the original analog signal while minimizing the amount of data to be handled, thus, it is necessary to set the sampling frequency as low as possible within a range over which the aliasing noises are not mixed with the original signal.
In accordance with the sampling theorem, the sampling frequency may be set at a level two times or more the maximum frequency in the original analog signal to prevent the waveform spectra thereof from being mixed with the aliasing noises. If the sampling frequency is set at a level two times the maximum frequency of an objective analog signal, the amount of data to be handled can be minimized so that the original analog signal can be faithfully reproduced.
FIG. 10 illustrates an example of the conventional analog signal synthesizing systems utilizing such PCM technique.
Such a system comprises a waveform memory 10 adapted to store a plurality of analog signals as PCM data which have been sampled with different sampling frequencies. PCM data corresponding to three channels are read out from the waveform memory 10 to synthesize an analog signal.
For example, if the analog signal synthesizing system is to be used to produce a synthesized sound combining a plurality of musical instruments, voice analog signals from the objective musical instruments, for example, guitar, drums and bass are previously stored in the waveform memory 10 as PCM data which are sampled with frequencies f.sub.s1, f.sub.s2 and f.sub.s3 corresponding to those of the musical instrument.
PCM data having the respective sampling frequencies f.sub.s1, f.sub.s2 and f.sub.s3 are read from the waveform memory 10 through the first, second and third channels and converted into analog signals through D/A converters 12-1, 12-2 and 12-3. These analog signals are then inputted into low-pass filters 14-1, 14-2 and 14-3, respectively. Aliasing noises are removed from the inputted analog signals by the respective low-pass filters 14-1, 14-2 and 14-3. Thereafter, the analog signals are applied to a mixer 18 through amplifiers 16-1, 16-2 and 16-3, respectively. At the mixer 18, the analog signals inputted thereinto and corresponding to three channels are mixed to form a synthesized analog from the three analog signals, for example, a synthesized analog sound waveform consisting of the sound waves representative of the guitar, drums and bass.
In order to use the low-pass filter 14 to remove the aliasing noises included in the PCM data, it is necessary to set the cut-off frequencies f.sub.c in each of the low-pass filters 14-1, 14-2 and 14-3 at a level one-half the sampling frequency f.sub.s as shown in FIG. 11A. This is because if the cut-off frequency f.sub.c is higher than 1/2 f.sub.s as shown in FIG. 11B, a part of the aliasing noises remains in the PCM data and will be reproduced as noises.
As described hereinbefore, however, the PCM data read from the waveform memory through the first, second and third channels are different from one another in the sampling frequencies f.sub.s1, f.sub.s2 and f.sub.s3. There is thus a problem in that the low-pass filters 14-1, 14-2 and 14-3 corresponding to the respective channels must be set at different cut-off frequencies f.sub.c1, f.sub.c2 and f.sub.c3.
In particular, such a conventional system is designed such that the sampling frequency of the PCM data read out from the waveform memory 10 is in one-to-one relationship with the cut-off frequency f.sub.c in the low-pass filter 14. Therefore, each of the channels is poor in universality. This takes place a problem in that the particular PCM data can be read out only from the corresponding channel, for example, the sound of a guitar from the first channel, the sound of drums from the second channel and the sound of a bass from the third channel.
When it is wanted to read many different PCM data, for example, ten or twenty different PCM data from the waveform memory 10, the number of channels corresponding to the number of the different PCM data portions must be provided. This results in a further problem in that the entire construction of the system becomes costly and more complicated.